1. Field of the Invention
The present invention relates to illumination devices and liquid crystal display devices, especially to an illumination device and a liquid crystal display device, each of which is stable in color temperature and luminance.
2. Description of the Related Art
Cold cathode fluorescent lamps (hereinafter referred to as “CCFLs”) have been conventionally used as backlights incorporated in back of liquid crystal panels of transmissive liquid crystal display devices used in laptop personal computers, computer monitors and television receivers. However, in recent years, due to the amelioration, among others, of the light efficiency of light-emitting diodes (hereinafter referred to as “LEDs”) and to the reduction of their cost, LEDs are increasingly being used as backlights of liquid crystal display devices.
LED backlight devices can either be of the direct type, where LEDs are arrayed below the back surface of a display panel of a liquid crystal panel or the like, or of the edge-light type, where a light guide plate is used. In general, the former is higher in efficiency in the use of light than the latter. The former also allows a reduction in weight.
LED backlight devices can, among others, either include an array of white LEDs that emits a white illuminating light, or include an array of LEDs of three colors, namely red (R), green (G), and blue (B), whose emitted lights are mixed to give a white light. It should be noted here that white LEDs are of a type that gives a white light by combining RGB fluorescent materials with a short-wavelength LED chip, of a type that generates a white light by combining a yellow fluorescent material with a blue LED chip, of a type that generates a white light as a mixture of lights emitted by LED chips of three colors (RGB), or of a type that generates a white light as a mixture of lights emitted by LED chips of two complementary colors.
In general, LEDs have characteristics such that their relative luminance tends to decrease with an increase in ambient temperature, i.e., have such a problem that their light efficiency changes depending on fluctuations in ambient temperature. In order to address this problem, the development of an LED backlight device which is not affected by fluctuations in ambient temperature and which maintains fixed light efficiency is in progress.
A specific example is disclosed in Japanese Patent Application Publication No. 2006-147373 A, discloses a backlight device including: a light source having a plurality of light-emitting diodes; a driving control section that drives the plurality of light-emitting diodes; and a temperature sensor that detects the temperatures of the light-emitting diodes. In the backlight device, a first preset upper limit temperature that is not higher than the maximum rated temperatures of the light-emitting diodes and a second preset upper limit temperature that is lower than the first preset upper limit temperature are preset in the driving control section. Japanese Patent Application Publication No. 2006-147373 A describes that the driving control section reduces an amount of driving electric current in cases where a temperature detected by the temperature sensor is not lower than the first preset upper limit temperature, fixes the amount of driving electric current at the present value in cases where the temperature detected by the temperature sensor is lower than the first preset upper limit temperature and higher than the second preset upper limit temperature, and increases the amount of driving electric current in cases where the present amount of driving electric current is lower than a preset value and in cases where the temperature detected by the temperature sensor is not higher than the second preset upper limit temperature. Further, Japanese Patent Application Publication No. 2006-147373 A mentions that the above-described configuration alleviates deterioration in characteristics of and/or failures in the light-emitting diodes used as the light source for the backlight.
Because such a conventional backlight device as disclosed in Japanese Patent Application Publication No. 2006-147373 A equally illuminates a whole area, there is a certain trend in variations in temperature among the LEDs. For this reason, the deterioration in characteristics of the LEDs can be reduced to a certain degree by presetting a temperature that is used as a benchmark when the driving control section controls the driving of the LEDs.
In recent years, however, area-active backlights have been attracting attention as illumination devices for use in display devices and the like. An area-active backlight is a backlight divided into small areas, thus allowing the control of the luminance of the backlight for each of the separate small areas in accordance with the gradations of an image displayed on a liquid crystal display device. In such an area-active backlight, the whole area is not equally illuminated; the emission of the light source (i.e., LEDs) is controlled for each area. In other words, for example, in the case of use of an area-active backlight in a display device, the LEDs of each area vary in electric power inputted thereto, depending on video signals. For this reason, the distribution of temperature within the backlight is always not constant, and varies depending on video signals. Accordingly, the following problem arises: namely, based on the preset temperature, a stable color temperature and a stable luminance cannot be maintained by merely controlling driving in accordance with a preset temperature. In other words, it is necessary to detect the temperature of each individual LED of each area in real time and to control the driving of the backlight device in accordance with that temperature.
However, a technique for appropriately detecting the temperature of each individual LED of each area in an area-active backlight and controlling the driving of the area active backlight in accordance with that temperature remains undeveloped.